1. Field of the Invention
The present invention relates to Orthogonal Frequency Division Multiplexing Access (OFDMA) communication systems, and more particularly to a method and an apparatus for subchannel assignment for suppressing and minimizing inter-antenna interference in an OFDMA system.
2. Description of the Related Art
With an increase of the requirements of a user in relation to internet service, the need for a communication system that can efficiently offer internet service is increasing. The existing communication network has been developed for the main purpose of a voice service but has drawbacks in that the existing communication systems have a relatively narrow data transmission band width, and require an expensive charge for its usage.
In order to resolve or correct such drawbacks, a study on a scheme of OFDM is being conducted as a representative example of a broadband wireless access scheme.
The scheme of OFDM corresponds to a typical transmission scheme employing multi-carriers and to a scheme that converts a symbol queue input in series into parallel data, modulates a converted symbol queue through multiple subcarriers having mutual orthogonality, and then transmit a modulated symbol queue. The above-mentioned scheme of OFDM can be widely applied to digital transmission technology that needs high-speed data transmission, such as wireless internet, Digital Audio Broadcasting (DAB) and digital television, Wireless Local Area Network (WLAN), and the like.
The scheme of OFDM (see for example, L. J. Cimini, “Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing,” IEEE Trans. Commn., vol. COM-33, no. 7, pp. 665-675, June 1985 and Richard Van Nee and Ramjee Prasad, “OFDM for Wireless Multimedia Communications,” Artech House, 2000.) corresponds to multiplexing technology that subordinately divides a bandwidth into multiple frequency subcarriers.
In OFDM, an input data stream is divided into several parallel substreams having a reduced data rate (therefore, the symbol length increases). Then, each substream is modulated, and is transmitted, on a separate orthogonal subcarrier. An increase of the symbol length improves the robustness of the OFDM against delay diffusion. OFDM modulation can be realized by efficient Inverse Fast Fourier Transforms (IFFT), which in turn enables multiple subcarriers having low complexity.
In the above OFDM system, channel resources employ an OFDM symbol in the time domain, and is enabled by using subcarriers in the frequency domain. Time and frequency resources consist of subchannels assigned to an individual user.
Also, the scheme of OFDM corresponds to a scheme of multiaccess/multiplexing, provides a multiplexing operation relating to data streams from multiuser to Up Link (UL) multiaccess employing a Down Link (DL) subchannel and an UL subchannel.
As previously described, the subcarrier is usually grouped into subsets called subchannels. For example, in a World interoperability for Microwave Access (WiMAX) system, the structure of OFDM symbol is made up of three kinds of subcarriers, including a data subcarrier for data transmission, a pilot subcarrier for an evaluation and synchronization, and a null subcarrier for a guard band and a DC carrier. An activated (data and pilot) subcarrier is grouped into subchannels.
A WiMAX OFDM physical layer (see for example, IEEE 802. 16-2004 (Revision of IEEE Std 802. 16-2001), “IEEE Standard for Local and Metropolitan Area Networks—Part 16: Air Interface for Fixed Broadband Wireless Access Systems,” October 2004 and IEEE 802. 16e-2005, “IEEE Standard for Local and Metropolitan Area Networks—Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems,” February 2006) supports subchannelization both in a DL (downlink) and in an UL (uplink), and a unit of the minimum frequency/time resources of the subchannelization corresponds to one slot.
Hence, research on an algorithm for assigning an adaptive subcarrier (subchannel) has been extensively carried out in a multi-user OFDM system. However, most of these algorithms are based on a Central Antenna based System (CAS).
A Distributed Antenna System (DAS) that is based on the OFDMA can allow a subcarrier to be used by another antenna.
In general, a DAS (see for example, A. M. Adel, A. Saleh, A. J. Rustako, and R. S. Ramon, “Distributed Antennas for Indoor Radio Communications,” IEEE Trans. Commun., vol. 35, pp. 1245-1251, December 1987 and S. Zhou, M. Xhao, X. Xu, J. Wang, and Y. Yao, “Distributed Wireless Communications System: a New Architecture for Future Public Wireless Access,” IEEE Commun. Mag., vol. 17, no. 3, pp. 108-113, March 2003) can provide macro-diversity that controls a large-scale fading and reduces an access distance by distributing antennas geometrically. The DAS has been introduced so as to solve a coverage area problem in an indoor wireless system, and afterwards has been applied to the performance improvement of a Code Division Multiple Access (CDMA) system.
FIG. 1 is a view illustrating a coverage area by each distributed antenna that is distributed around a Base Station (BS) centered in a DAS. With reference to FIG. 1, in the DAS, the antennas of the BS are uniformly distributed geometrically, and with each antenna of the BS as the center of a hexagon area, an overall area can be divided into six hexagonal subareas. If an average access distance decreases in the DAS, as transmission power is reduced, inter-antenna interference diminishes, and capacity can increase. Channel conditions of the antennas of the BS are measured and analyzed by a subscriber station (SS) in each frame, and then an antenna M having the maximum gain can be selected as a serving antenna in the next frame. The value of M is equal to or greater than ‘1.’ Herein, the value of M is confined to being ‘1’ to have a positive value.
If the number of antennas equals ‘P’ within a cell of the DAS, the number of developed subcarriers becomes P times as many as a CAS. Thus, an assignment of resources developed more complicated in the DAS.
At present, in the DAS based on the OFDMA, an algorithm for assigning subchannels can be classified into several kinds as in the following.
1. Each antenna develops all subchannels.
2. All subchannels are assigned to cells only once. This implies that if any subchannel is used by one antenna in a cell, the subchannel cannot be employed even by any other antenna within the cell.
3. Each subchannel is assigned from a global viewpoint, and in order to obtain diversity gain, it is allowed for two adjacent antennas to use one SS through the same subchannel.
However, if each antenna develops all subchannels as described above, this is the same as cell division from a standpoint of frequency reuse, and incurs inter-antenna interference similar to co-channel interference in the cell division. Also, if all subchannels are developed by one remote antenna and one SS, even though interference is excluded from another antenna, this is a waste of bandwidth, and problems arise, for example in hot-zones.
Hence, at present, even though two antennas are sufficiently far away from each other in the OFDMA-based DAS, its subchannels cannot be reused.
In this manner, if each antenna develops all subchannels, this is the same as cell division from a viewpoint of frequency reuse, and causes such problem that a serious inter-antenna interference similar to co-channel interference is incurred.